Microfilm reader/printer zoom lens system

ABSTRACT

A zoom lens system having a large magnification is particularly useful in micrographic reader/printers, and any type of projection lens system. The lens comprises at least three components. The first component is provided with a fixed aperture stop. The other two components are axially movable for zooming.

CROSS-REFERENCE APPLICATIONS

This is a divisional application of U.S. application Ser. No. 361,466,filed Jun. 5, 1989, now U.S. Pat. No. 5,132,846.

FIELD OF THE INVENTION

The present application relates to variable focal length lenses and inparticular to lens systems for use with micrographic projectors andprinters.

DESCRIPTION RELATIVE TO THE PRIOR ART

In order to conserve space, documents are often stored photographicallywith their size greatly reduced on microfilm, microfiche, or othermedia. Such processes and media are known generally by the termmicrographics.

In order to utilize a document which has been stored in micrographicformat, a projector or reader is required. Such a reader will produce anenlarged reproduction of the reduced image on a screen for viewing bythe operator. Often such readers also include a printer. When a desireddocument has been located by projection onto the screen, the operatormay produce a permanent enlarged copy through photographic or plainpaper copier techniques. Systems which permit such reading and printingare known as reader/printers. The term reader/printer will be usedherein to denote both readers containing only a projector and thoseincluding a printer in addition to a projector.

A problem which arises in the use of such systems relates to the factthat different reduction factors may be used in recording themicrographic images and that recorded documents may be of differentsizes. As a result, different magnifications are required in order toreproduce properly such images or to make a reproduced image fill adesired area. In order to accommodate such variations, micrographicreader/printers are commonly provided with lenses having a variety ofmagnifications. In some cases, as many as fifteen or more lenses arerequired to accommodate all possible enlargement factors that may bedesired. The frequent changing of lenses can create a greatinconvenience to operators of such equipment and can increase the timerequired for reading and printing documents stored in micrographic form.

U.S. Patent Nos. 4,750,820; 4,743,102; 4,746,204 and 4,733,951 eachdisclose a zoom lens as a solution to the above problem. Because eachzoom lens provides a variety of magnifications, at most only a few zoomlenses are needed to cover the entire desired magnification range. Zoomlenses disclosed in U.S. Pat. Nos. 4,750,820; 4,743,102 and 4,733,951have an aperture stop located between the two zooming groups. This typeof design usually causes as much as 50% or more vignetting of the lightin the corners of the field relative to the axis. With such grossamounts of vignetting, depending upon the angular coverage, the relativeillumination of such optical systems can fall to 30%. This is verytypical of camera systems. When this occurs in microfilm systems, theobserver is forced to stop down the lens system to eliminate theannoying visual effect. However, when this is done the overallillumination dictated objectionable exposure durations. In areader/printer a zoom lens is commonly located behind a collimator andDove prism rotator assembly, which is used to rotate the image, and itis obviously important to minimize the size of this assembly.Accordingly, the lens elements near the rotator assembly should be keptsmall, which can be accomplished by locating the aperture stop near thatassembly. U.S. Pat. No. 4,733,951 has an aperture stop located in frontof the two zooming optical units. Since the two optical units are afront positive optical unit and a rear negative optical unit, this is atelephoto-type lens. A telephoto lens generally results in a short backfocal length compared to its effective focal length. Because intelephoto-type lenses back focus gets smaller with higher magnification,the back focus can impose an upper limit on the obtainablemagnification. In addition, having film immediately adjacent to lenselements is inconvenient because of supporting difficulties and,because, if located next to the focal plane, the lens elementsthemselves need to have good scratch and dig tolerances becausescratches and digs in the lens surface will be imaged on the focalplane. For the same reason, if dust or dirt gets in the back of such asystem, this will be seen in the image; which require these lenselements to be kept very clean.

U.S. Pat. No. 3,724,927 also discloses a zoom lens system designed formicrographic applications. However, this is a complicated systeminvolving three separate lens assemblies and requires a very long lenswith many lens elements, which is inconsistent with the desirability forcompactness.

The present invention overcomes the foregoing problems by means of areader printer zoom lens for use in conjunction with an image rotatorassembly to provide a wide range of magnification; minimum vignettingand constant, high illumination; a constant object-to-image distance; ahalf field coverage greater than 10 degrees; and advantageousutilization of a non-collimated light entering an aperture stop. Also,the optical elements located near the rotator assembly are of relativesmall aperture and the size of the optical components of that assemblyis also correspondingly small.

Other advantages and novel features will become apparent from thefollowing description of illustrative embodiments of the inventionillustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention, andtogether with the description, serve to explain the principles of theinvention.

FIG. 1 is a schematic representation of a lens system according to afirst illustrative embodiment of the present invention;

FIGS. 2 and 3 are tables of constructional data for the firstembodiment, illustrated in FIG. 1;

FIG. 4 is a schematic representation of the second illustrativeembodiment of the present invention;

FIGS. 5 and 6 are tables of constructional data for the secondembodiment, illustrated in FIG. 4;

FIG. 7 is a schematic representation of a third illustrative embodimentof the present invention;

FIGS. 8 and 9 are tables of constructional data for the thirdembodiment, illustrated in FIG. 7.

FIG. 10 is a schematic representation of a fourth illustrativeembodiment of the present invention; and

FIGS. 11 and 12 are tables of constructional data for the fourthembodiment illustrated in FIG. 10.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE PREFERRED EMBODIMENTS

The lens systems of the present invention provides variablemagnification or zoom lenses that are especially advantageous for use inmicrographic reader/printers. All four illustrative embodiments of thepresent invention are adapted for use with a collimator and prismrotator assembly and have the same object-to-image distance when usedwith such an assembly. Thus, at most, only a small number of zoom lensesis required to view or print micrographic images having widely differentreduction factors. The location of the aperture stop towards the frontof the lens system described in the following embodiments minimizesvignetting and increases illumination while minimizing the size of theoptical assembly in front of the lens system, e.g. a collimator andprism rotator assembly. Having two moving optical units of positivepower further improves compactness of the lens system while providing arelatively large back focal distance.

I have found that, with two movable optical units, aberrations arecontrolled to acceptable levels and a constant object-to-image distanceis maintained, when used with a collimator/prism assembly. It isadvantageous to have both zooming optical units of positive powerbecause a negative-positive combination of optical units tends to makethe system long and a positive-negative combination of optical unitsgenerally results in systems with a short back focus. Having twopositive moving optical units provides a compact lens system with alonger back focus than would be the case with positive-negative opticalunit combination. A negative-negative combination of optical units makesit difficult to converge light toward an image plane.

When used with the collimator and prism rotator assembly, this inventiontakes advantage of the fact that light need not be actually collimatedwhen transversing the Dove prism, which allows the lens system to haveequal object-to-image plane distances without resorting to telephoto andinverted telephoto designs operating in a fixed stop-to-image distance.

According to this invention, a zoom lens, also herein referred to as a"lens system", comprises, from front-to-rear, a first fixed optical unithaving an aperture stop, a second optical unit axially movable forchange in magnification, a third optical unit axially movable for changein magnification and for focus control, and, in some of the illustrativeembodiments, a fourth optical unit which is fixed. In all of theembodiments the zooming spaces have been desensitized to aberrationdeterioration with zoom motion.

The first zoom lens embodiment illustrated in FIG. 1, comprises fouroptical units. The lens system includes a first, front fixed opticalunit A, provided with a fixed aperture stop 15, a second optical unit Bthat is movable for zooming, a third optical unit C that is movable forzooming and focus control and a fourth, rear fixed optical unit D. Aprotection plate P is provided behind the four optical units; however,the back focal length shown in FIG. 3 is measured from the rear vertexof the last lens element and does not include the effect of the plate P.Such a plate does not constitute an "optical unit" for purposes of thepresent application. The first optical unit A is located nearer ortowards the object plane. The movement of the second optical unit B isprimarily responsible for the change in magnification. While themovement of the third optical unit C also effects the change inmagnification, the third optical unit C is primarily responsible inmaintaining a fixed image plane position. The lens system is sostructured, that the effective focal length of the overall lens systemis varied by changing, relative to each other, distance S₁ between thefirst and second optical units A and B, distance S₂ between second andthird optical units B and C, and distance S₃ between the third andfourth optical units C and D. This lens system varies from the focallength of 19 mm. and f/1.97 to a focal length of 25 mm. and f/2.59, thusproviding a 50× to 38× magnification range and a 1.315:1 magnificationratio while maintaining a fixed object to image distance when used withthe collimator and prism assembly.

The first front optical unit A has negative refractive power andcomprises an aperture stop 15 and a negative biconcave lens component 11having front and rear surfaces R₁ and R₂ and thickness T₁. The values ofthe radii of surfaces R₁ and R₂ and the value T₁ as well as the radii ofcurvature and thicknesses of the rest of the elements are given inTable 1. The aperture stop 15 is fixed and has a clear aperture diameterof 9.64 mm.

The second optical unit B is spaced from the first optical unit A by avariable air gap S₁. The second optical unit B has positive refractivepower and includes two positive lens components 22 and 24. Lenscomponent 22 is a biconvex lens element having front and rear surfacesR₃ and R₄ and thickness T₂. Lens component 24 is a positive cementeddoublet that includes a biconvex lens element 25 and a meniscus lenselement 26. The lens element 25 has a front surface R₅, a rear surfaceR₆ and a thickness T₃. The meniscus lens element 26 has front and rearsurfaces R₆ and R₇, respectively, and thickness T₄.

A third optical unit C is spaced from the second optical unit B by avariable air space S₂. The third optical unit C has positive refractivepower and includes a negative, cemented doublet lens 32 and a positivesingle lens 36. Lens component 32 includes a meniscus lens negativeelement 33 and a positive meniscus lens element 34. The lens element 33has front and rear surfaces R₈ and R₉, respectively and thickness T₅.The lens element 34 has front and rear surfaces R₉ and R₁₀, respectivelyand thickness T₆. Lens component 36 is a biconvex lens element havingfront and rear surfaces R₁₁ and R₁₂ and thickness T₇.

The fourth optical unit D is spaced from the third optical unit C by avariable air space S₃. The fourth optical unit D is negative andincludes a positive single lens component 42 and a negative cementeddoublet 44. The radii of curvature for the lens 42 are R₁₃ and R₁₄, andits thickness is T₈. Lens component 44 consists of a biconvex positivelens element 45 with radii of curvature R₁₅ and R₁₆ and a biconcavenegative lens element 46 with radii of curvature R₁₆ and R₁₇. Lenselements 45 and 46 have thicknesses T₉ and T₁₀ respectively.

The variation in space S₁ between optical units A and B contributes tothe focal length change about two times more than an equal increment ofvariation in space S₃. The actual magnitude of the ratios depends on thezoom position. Besides changing the focal length, the increments ofchange in spaces S₁, S₂ and S₃ also partially compensate foraberrations. For example, some residual negative astigmatism that isintroduced as zoom space S₁ increases is compensated by a change in zoomspace S₃.

FIG. 4 illustrates a second embodiment of the present invention, whichis generally similar in construction the the first embodiment describedabove and illustrated in FIG. 1. FIGS. 5 and 6 are tables giving thevalues for the various parameters. In FIG. 4, the four optical units aregiven the same reference letters, but with a prime (') suffix. Likewise,the variable spacings are given the same reference, but with a singleprime suffix added. However, because the shape of lens elements and thenumber of lens elements in each optical unit differ from one embodimentto another, distinct reference numerals are used for each lens elementin each of the embodiments.

As in the case of the first embodiment, in this second embodiment thefront optical unit A' is a biconcave negative lens element, and the rearoptical unit D' includes of a front, positive single lens component, anda rear, negative cemented doublet. As before, the cemented doubletconsists of a front biconvex positive lens element and a rear biconcavenegative lens element.

The construction of the second optical unit B' and the third opticalunit C' are markedly different from the construction of the opticalunits D and C in the first embodiment. Unit B' is a positive tripletderivative by which is meant a lens of a positive triplet type (+-+).For example, such lenses may contain two positive lens components infront, followed by a negative lens component and then a positive lenscomponent. B' has a basic shape of a positive triplet lens, but with thecentral negative lens element 55 cemented to the rear positive lenselement 56. Group C' is also a positive triplet derivative, including afirst plano convex positive lens element 57; second, biconcave negativelens element 58; third, biconcave negative lens element 59; and fourth,biconvex positive lens element 60. This particular triplet derivativeoriginated from splitting a central negative lens element of an ordinarypositive triplet lens into two negative lens elements.

The ratio of extreme focal length of the second embodiment is 40.1:23.1or 1.74:1. The lens of the second embodiment provides 40× to 23×magnification range when used with the collimator and prism assembly.The diameter of the aperture stop is 9.69 mm. and, as before, the lenslength, the back focal length and the distance from object-to-image whenused with a collimator and rotating prism are maintained constantthroughout the zooming range.

FIG. 7 illustrates a third embodiment of the same invention and FIGS. 8and 9 are tables giving the values of the various parameters. The ratioof extreme focal length of the third embodiment is 66:31 or 2.1:1 andmagnification range is 30× to 14× when used with the collimator andprism assembly. In FIG. 7 the four optical units are given the samereference letters as their counterparts in the first embodimentdescribed above and illustrated in FIG. 1, but with the addition of adouble prime (") suffix. Likewise, the variable spacings are given thesame reference, but with a double prime suffix added.

In this third embodiment, the first, front optical unit A" includes twolens components 70 and 71. Lens component 70 is a negative meniscus lenselement and lens component 71 is a positive meniscus lens element. UnitB, includes four lens components: a positive cemented lens component 73,a positive meniscus lens component 76, a positive meniscus lenscomponent 77, and a biconvex negative lens component 78. The positivecemented lens component 73 consists of a biconvex positive lens element74 and a negative meniscus lens element 75. Unit C" consists of a singlebiconvex lens element 80, and optical unit D" consists of a single,plano-concave lens element 82. While in this third embodiment, opticalunits C" and D" are two distinct and separate optical units, opticalunits C" and D" could be merged into one single moving optical unit,resulting in a configuration of three optical units, one of which isfixed and two of which are moving. This is illustrated in the embodimentdescribed below.

FIG. 10 illustrates the fourth embodiment of the invention and FIGS. 11and 12 are tables giving the values of the various parameters. In FIG.10 the optical units are given the same reference letters as theircounterparts in the first embodiment described above and illustrated inFIG. 1, but with the addition of a triple suffix. Likewise, the variablespacings are given the same reference characters, but with a tripleprime suffix added.

In this fourth embodiment, the first three optical units A'", B'" andC'", are similar to the first three optical units A", B" and C" of thethird embodiment, except that there is no corresponding optical unitD'".

As in the case of a third embodiment, the optical unit A'" includes anegative meniscus lens component 90 and a positive meniscus lenscomponent 92, the optical unit B'" includes four lens components 94, 96,98 and 100 and optical unit C'" consists of a single positive lenscomponent 102. As before, optical unit B'" includes a first positivecemented lens component 94; a second, positive meniscus lens component96; a third positive meniscus lens component 98 and a fourth, biconvexnegative lens component 100. The positive cemented lens component 94 inoptical unit B'" consists of a biconvex positive lens element 93 and anegative meniscus lens element 95.

While in the above-described embodiments, the fixed aperture stop islocated towards the back of the first optical unit, it is to beunderstood that the aperture stop may be located near the front of thefirst optical unit, or between the elements included in the first fixedoptical unit. Furthermore, it may be possible that the first, fixedoptical unit A consists of an aperture stop only. In this case theinvention will always comprise optical units A, B and C and the powersof a second optical unit and a third optical unit will be positive.

While the general zoom lens configuration of the present invention iscommon to the four illustrative embodiments; i,e., two moving opticalunits following a fixed optical unit provided with a fixed aperturestop, the shape of the lens elements and the number of lens elements ineach optical unit differ from one configuration to another. Forillustrative purposes the following three examples are given below:

1. The second optical unit in the first embodiment consists of two lenscomponents and three lens elements, while the second optical unit in thesecond embodiment consists of four lens components and five lenselements.

2. The last lens element in the second optical unit is negative in thefirst embodiment, but is positive in the third embodiment.

3. The third optical unit of the third embodiment consists of fourair-spaced single lens elements, but, the third optical unit of thesecond embodiment consists of a single positive lens element.

While the above-mentioned magnification ratios of illustratedembodiments of the invention are 1.315:1; 1.736:1 and 2.13:1, it shouldbe obvious that the invention may be embodied successfully in zoomlenses having different magnification ratios as well. Similarly, itshould be understood that the focal lengths and other system parametersof the illustrative embodiments can be scaled up or down for differentapplications.

Although this invention is particularly useful in micrographicreader/printers, it is also applicable to photographic or otherobjectives. It can be used as a stand-alone lens system, or be used inconjunction with an add-on front lens system, and as such, will varymagnification ratio or object-to-image distance.

In the embodiments specifically described above all of the surfaces arespherical, but it is to be understood that other embodiments of thisinvention may have nonspherical surfaces.

The invention has been described in detail with particular reference toillustrative preferred embodiments, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

What is claimed is:
 1. A zoom lens for forming an image of a substantially collimated light output by a collimator, said zoom lens having an optical axis and comprised of a plurality of lens components centered on said optical axis and arranged in at least three optical units, said lens units having powers and spacings to image said collimated light at an image plane, two of said lens units are movable to vary the focal length of said zoom lens, said zoom lens including: a frontmost, fixed optical unit of negative optical power for receiving said substantially collimated light and provided with a fixed aperture stop; a second optical unit that is axially movable for zooming located behind said front optical unit, said second optical unit having positive power; and a rear optical unit immediately following said second optical unit that is axially movable for zooming located behind said front optical unit and said second optical unit, said rear optical unit having positive power.
 2. A zoom lens according to claim 1, further including an image rotator assembly.
 3. A zoom lens according to claim 1, further including collimating means for collimating light image modulated by a micrograph, said collimating means being located in front of the first optical unit.
 4. A variable focal length lens system for forming an image at an image plane of a substantially collimated light output by a collimator in a micrographic apparatus, said lens system comprising three optical units, namely;1) a first, fixed optical unit including a fixed aperture stop, said first unit being adjacent to said collimator; 2) a second, axially movable optical unit to provide zooming action, said second optical unit having positive power; and 3) a third, axially movable optical unit to provide zooming action, said third optical unit having positive power, said lens units having powers and spacings to image the collimated light at the image plane.
 5. A variable focal length lens system according to claim 4, wherein said first optical unit is negative.
 6. A variable focal length system according to claim 5, further including an image rotator assembly.
 7. A variable focal length system according to claim 4, further including an image rotator assembly.
 8. The variable focal length lens system according to claim 4, further including collimating means for collimating light image modulated by a micrograph, said collimating means being located in front of the first optical unit.
 9. A variable focal length lens system for forming an image of a substantially collimated light for use in micrographic projectors and printers, said lens system consisting three optical units, namely:1) a first, fixed optical unit including a fixed aperture stop; 2) a second axially movable optical unit to provide zooming action for receiving said substantially collimated light and, said second optical unit having positive power; and 3) a third, axially movable optical unit to provide zooming action said third optical unit having positive power.
 10. A zoom lens according to claim 9, wherein said front fixed optical unit has negative power. 